Current Public Land Mobile Networks (PLMN) typically comprise a radio access network and a core network. The radio access network is responsible for providing wireless access to network users, whilst the core network is responsible, inter alia, for subscriber service access control and subscriber roaming. In the case of 2G or GSM network, a typical PLMN comprises circuit switched (CS) core network for handling voice calls and a packet switched (PS) core network for handling data. A 3G or UMTS network similarly comprises PS and CS core networks, whilst a 4G or LTE network comprises only an Enhanced Packet Core (EPC) core network. In addition, service networks may co-exist with the PS and CS core networks. One such service network is the IP Multimedia Subsystem (IMS) network. The IMS network may also be considered as a core network. In practical implementations, subscribers may roam between different radio access networks, and services may be switched between core networks, e.g. a voice call may be switched from a 3G radio access network and CS core network to a 4G network, with the same IMS core network remaining in control of the session.
PLMN operators are always keen to reduce network signalling load for a given subscriber volume, to increase network resilience to failure, and to reduce the complexity and number of specialist network nodes. With this in mind, the 3GPP organisation has defined a so-called Data Layered Architecture (DLA) that splits the traditional node and network architecture introduced for networks such as GSM, UMTS and EPC in two halves; an application front end (Application-FE) that provides the logic for a node, and a Data back end (Data-BE) that provides data. This is illustrated in FIG. 1. The Data-BE provides a highly available and geo-redundant solution to the Application-FEs that simplifies the design of applications. DLA also simplifies the routing in the core networks since any node can, if it is built as a stateless FE, serve any subscriber. The principles and advantages of DLA can be summarised as follows:                Simplified routing to Application FEs in Core Networks                    Any Application FE can serve any subscriber                        Data & Repository Consolidation                    Several applications use the same backend (repository consolidation)            Data de-duplication by allowing several applications to access the same data object (data consolidation)                        Scalable and Highly Available system                    Backend provides HA and Geo Redundancy                        Simple scaling of Application Front Ends        Simplified Application Design                    Provisioning into back end database            Dynamic allocation of subscribers to Application FEs            Allows for stateless Application FE design            Data Availability provided by the Backend                        
Consider the Home Location Register (HLR) that is implemented in 2G and 3G core networks and that acts as a central database for subscriber information. The HLR stores details of subscriptions issued by the network operator. Typically, a network will comprise a number of HLRs, each of which is assigned to a group of network subscribers. An HLR provides subscriber information to other core network nodes, such as the 2G Mobile Switching Centre (MSC). In the traditional architecture, core network applications, such as the MSC, must identify and route signalling to the correct HLR (i.e. the HLR currently serving a given subscriber). According to the DLA architecture, an HLR-FE can serve any subscriber (i.e. all network subscribers) and network routing becomes very simple: the advanced Data-BE performs routing for the applications.
Other advantages of DLA include Data and Repository Consolidation allowing databases and data-object sharing, and simplified provisioning (and removal) of subscribers within the system.
While the Data-BE is designed to provide a highly available, resilient and redundant service to the Application-FEs, extreme failure conditions may occur. A Data-BE is partitioned into groups of subscribers that are divided between the nodes that constitute the entire Data-BE. These groups are referred to herein as Data Groups (DGs). Each DG is replicated between different Data-BE nodes for redundancy. If a first Data-BE node where the master version of a particular DG resides becomes unavailable, another Data-BE nodes takes over from the first Data-BE node using a synchronized, up-to-date copy of the DG.
In some circumstance, the entire DG might become unavailable. In this case, both the master and slave versions of the DG at all Data-BE nodes holding a copy of the DG are unavailable. During recovery from these extreme circumstances, the information for the subscribers belonging to the affected DG must be recreated.
Subscriber data includes substantially static subscriber data and dynamic subscriber data. Static subscriber data includes data that is never or rarely updated, for example, contact details, address details and so on. Dynamic subscriber data includes data that is more frequently updated, such as subscriber location, communication sessions a subscriber is involved in, whether the subscriber has set a call forwarding rule and so on.
In known, monolithic HLR systems, loss of availability of subscriber data requires sending Reset messages to all Core Network nodes in order to recreate the data. This is particularly important for recreating the dynamic subscriber data. Each subscriber is assigned to an HLR, which has an identifier known as an HLR-No. A receiving Core Network node (e.g. an MSC/VLR or Serving GPRS Support Node, SGSN) identifies the affected subscriber by the HLR-No, and remove the associated records. Removal of the associated records forces a location update for each removed subscriber served by the HLR identified by HLR-No, and data that was lost is gradually built up again.
However, the above solution to loss of HLR is not practical in a DLA network. In a DLA network, all subscribers are served by one “virtual” HLR, consisting of stateless HLR-FEs, with one common HLR-No for the entire system. A Reset using HLR-No for all HLR-Fes would remove every subscriber in the entire network, which would lead to a loss of data and a very large increase in signalling to gradually build up the data once more.
The MAP specification (3GPP 29.002) allows a list of HLR-Ids to be sent from the HLR to a CN node together with the HLR-No, where the HLR-Id includes “the leading digits of IMSI” (for example MCC+MNC+MSIN). This allows a number series to be removed in a core network node. This could be modified to reset subscribers in core network nodes if each DG in a Data-BE is assigned subscribers on a per number series basis. However, this is also undesirable as the provision of subscriber data does not naturally lend itself to an ordered sequence of numbers, as this would reduce flexibility in how operators can assign subscribers to different DGs.